The invention concerns installations for making plastic laminates with metal laminae especially for printed circuits, including the multi-layer laminates.
Plastic laminates for printed circuits are normally made by forming packages consisting of sheets of pre-preg, associated to heat and pressure, with a sheet of metal on one or on both faces of the package.
To increase production a hot press is used to obtain several plastic laminates at each working cycle, the packages being piled up with steel flattening sheets in between them. Automatic loading means are used to place the piles on the plates of a suitable press for providing pressure and heat simultaneously.
At the end of the working cycle, lasting 60′-90′ at temperatures up to 180° C. and at pressure levels of from 10 to 50 kg/cm2, the packages are transferred from the hot press to a cold press for the cooling cycle, at the end of which an automatic unloading means removes them and despatches them to a package separating line. Here the plastic laminates are separated from the flattening sheets which are cleaned and transferred to the packaging department.
At the end of the heating cycle lasting over 100′, including cooling at 30-50° C., a compact and rigid product is obtained with all its component parts closely associated. Presses suitable for such production requirements are complex due to the number of plates, to the need for simuultaneous production of heat and pressure by conduction, with well-defined and precisely functioning cycles, at temperatures as uniform as possible in the various packages forming the pile; obviously however only those at the top and bottom will be in direct contact with the heat-generating plates.
Propagation of heat from the hot plates to the piled up packages, especially to those immediately above and below the packages in contact with said hotplates, is greatly hindered by the heat having to pass through the various layers of prepreg which, consisting as they do of paper, fiberglass and plastic materials, are very poor conductors of heat.
Cooling each package at the end of the cycle will similarly be hindered by the compactness of the piles of packages.
The presence of so many plates renders more complex the structure of the press, as well as loading and unloading of the packages.
To overcome these drawbacks, a few years ago the same inventor disclosed a process for endothermic heating of the packages using the metal laminae themselves, especially copper laminae, to generate heat by connecting them to a suitable source of electricity thereby transforming them into electric resistances.
On terminating the heating cycle lasting 40′-60′, the process continues as described above.
This process offers appreciable advantages especially in generating and diffusing heat as the means required are extremely simple, but above all due to elimination of problems at present existing in the use of hot presses, and obtaining, within a short space of time, penetration of the heat to the packages closest to the centre of the pile. As endothermic heating signifies that heat is actually generated at the level of each package, they all reach a uniform temperature within a short time which also signifies a shorter production cycle and higher quality products.
The drawback in making up the packages however remains, especially because a strip of copper, or of another highly conductive material, has to be wound serpentinewise round the package formed of sheets of prepreg.
In certain special cases, this composition may become complex and irregular in view of the size of the laminates and of the need for ever greater accuracy in the production of printed circuits and multi-layer laminates.
Another problem arises over separating the packages in the pile as the copper strips passing from one package to another must be cut and trimmed.
It will be clear from the above that, in both processes, the funda-mental part, namely the press which provides pressure and heat, assumes a role of almost secondary importance in terms of both cost and time.
The present disclosure overcomes the drawbacks referred to above and offers other considerable advantages as will now be described.